Although an injection molding machine may be controlled using preestablished settings of parameters such as hydraulic pressure to the ram, material temperature and cycle time, it has been found that to produce molded parts of consistent uniform quality it is necessary to provide for continued monitoring of process variables and to effect correction as required.
A common control technique is to monitor cavity pressure throughout the molding cycle. By establishing a pressure profile which is representative of an "ideal" cycle during which a molded part of desired quality is produced and using such profile as a reference, process conditions during subsequent cycles may be compared and corrective action taken to cause the subsequent cycle pressure profiles to conform to the reference profile. Although it is recognized that the more promptly corrective action is taken in response to a sensed error condition, the greater the uniformity of repetitive cycles it has previously been possible to take effective corrective action only in a subsequent machine cycle. Some process control techniques make reference to control during the same cycle by altering conditions within the same machine cycle. These actions usually amount to resetting a valve which is no longer controlling the active part of the process. Such systems provide effective control of process parameters only during the subsequent cycle.